1. Field of the Invention
The present invention relates to a viscous damper, and more particularly to a viscous damper which is used as a damper in a power transmission device that drives accessories such as an alternator and the like by means of a gearing directly coupled with a crankshaft through the damper.
2. Description of Background Art
For a power transmission device in conventional vehicles, a power transmission device using a viscous damper has been well known. An example is a power transmission device having a construction in which the rotational driving force of a first flywheel directly connected to a crankshaft is transmitted to a second flywheel via a torsion spring and a viscous damper, and the rotational driving force transmitted to the second flywheel is transmitted to an output shaft via a clutch coupling. In this power transmission device, when a relative vibration between the two flywheels is relatively stable and small in amplitude in a high rotational speed range, the torque is transmitted primarily by the torsion spring. When the relative vibration between the two flywheels is large in amplitude, that is, when a relative torsional vibration with relatively large amplitude is generated between the first flywheel and the second flywheel in a low rotational speed range at or below an idling speed, the viscous damper works to dampen and absorb the torsional vibration and to accomplish a smooth power transmission of the rotational driving force, which consequently reduces the vehicle body vibration at the low rotational speed range (See for example Japanese Patent Publication No. 3,206,997 (Page 4, FIG. 1, FIG. 6).
A power transmission device having a viscous damper described in the invention disclosed in Japanese Patent Publication No. 3,206,997 is configured as follows. As shown in FIGS. 7(a) and 7(b), to a first flywheel 02 which is directly connected to the crankshaft 01 of an internal combustion engine, a second flywheel 03 is coupled with a torsion spring 04 which is a first damper means and a viscous coupling 07 interposed inbetween. The rotational driving force of the crankshaft 01 transmitted to the second flywheel 02 is transmitted to an output shaft 06 via the coupling operation of a clutch 05.
For the torsion spring 04 in the viscous damper, an elastic member such as a coil spring and the like is used. The viscous coupling 07 comprises multiple driving side plates 08, driven side plates 09, and floating plates 011 which are alternately arranged in a coupling space 010 filled with a viscous fluid. The viscous torque on account of a relative movement of these plates with the viscous fluid acting therebetween dampens and absorbs the vibration energy due to the rotational driving force.
With the viscous damper in this power transmission device, the driving force is transmitted between the two flywheels via the torsion spring which is the first damper means when the relative vibration between the two flywheels of the first flywheel and the second flywheel is relatively stable and small in amplitude in a high speed rotational speed range. When the relative vibration between the two flywheels of the first flywheel and the second flywheel is relatively large in amplitude, that is, when a torsional vibration with large amplitude is generated between the both wheels in a low rotational speed range at or below an idling speed, the viscous coupling which is the second damper means works to dampen and absorb the torsional vibration and to suppress the vibration in the power transmission device in a low speed rotational speed range, which consequently reduces the vehicle body vibration and noise.
However, for the viscous damper in the aforesaid power transmission device, the damper constitution uses the following two means together; an elastic member such as a torsion spring is used as the first damper means, and a viscous fluid is used as the second damper means. In a viscous damper of such a construction, an occurrence of resonance phenomena by the torsion spring or the elastic member as the first damper means is unavoidable. The occurrence of resonance phenomena causes new vibration and noise in the viscous damper or the power transmission device using the viscous damper, resulting in a failure to accomplish a sufficient suppression of vibration and noise in the viscous damper or the power transmission device. Further, the presence of a plurality of sources causing vibration and noise in the power transmission device using such a viscous damper increases the difficulty in designing the power transmission device because the vibration and noise sources are affecting each other in a complex manner.
Under afore-described circumstances, with regard to a power transmission device for driving accessories by the crankshaft, on the premise that pulsating rotational torque changes which are transmitted from the crankshaft to the accessories through the power transmission device are effectively dampens and absorbs, and the smooth driving of the accessories is ensured, a simplified structure of the power transmission device is maintained by interposing a simplified viscous damper in the power transmission device. Incidentally, a provision of an improved structure for the viscous damper, which is used in the power transmission device for driving accessories, and is capable of effectively dampening and absorbing rotational torque changes from the crankshaft has been desired.